The quiet running properties (noise) of greases used to lubricate deep groove ball bearings have become increasingly important to bearing manufacturers in their selection of factory fill greases. Historically, bearing manufacturers became increasingly concerned about bearing vibration that manifested itself as audible sound as the demand grew for quieter machines. As bearings were machined to finer tolerances, becoming inherently less noisy, the noise contributions of the greases used to lubricate them became increasingly apparent. Consequently, the major bearing manufacturers independently developed instrumentation that allowed measurement of the contribution of grease to bearing noise. In addition, correlation of bearing life to the presence of contaminants promoted an even greater concern with grease noise testing because the assumption is often made that grease noise always correlates to the presence of contaminants and therefore with shortened bearing life. Although most grease manufacturers would agree that knowing the noise characteristics of a grease does not provide sufficient information to allow prediction of the life of a bearing lubricated with it, noise testing is nonetheless increasingly used to assess the overall quality of ball bearing greases. Grease manufacturers therefore must be concerned with the noise quality of their products and with the various methods by which grease noise quality is determined if they are to continue to supply greases to the bearing manufacturing industry.
Although grease noise testing has been the subject of numerous publications over the past twenty-six years, no standard test instrument, test bearing, or test protocol has been adopted by either grease suppliers or bearing manufacturers during this time. In fact, a wide variety of proprietary grease noise testing methods is currently in use, particularly in the bearing manufacturing industry, where each major bearing manufacturer has developed its own proprietary instrumentation and methods. In addition, each method is considered by its proponents to provide a competitive edge for the company that uses it.
Because of the above considerations, testing the quiet running (noise) properties of grease has been an issue. Originally, a manual test was developed which allowed assessment of the running properties of a batch of grease by the feel of a bearing packed with it. As the noise quality of bearings themselves improved, it became necessary to be able to detect lower and lower levels of bearing vibration. As a result, Chevron Research (Richmond, Calif.) began using a modified bearing vibration level tester (an anderometer) to test for grease noise and began carefully studying the effects of additives and processing variables on grease noise. The anderometer, which was originally developed to assess bearing vibrational quality, measures the radial displacement of the outer race of a bearing as a function of its rotation. In fact, the name anderon is an acronym for “angular derivative of the radial displacement”. In physical terms, the anderon is expressed as displacement distance/unit rotation:1 anderon=0.62 microinches/radian.The sensor head, which is in contact with the outer race, detects bearing vibration. The sensor signals are amplified and filtered into three frequency bands which span the range of audible sound frequencies:
Low:50-300 HzMedium:300-1,800 HzHigh:1,800-10,000 Hz.
Vibration (noise) due to grease can be detected in the medium and high frequency bands. In the earliest version of the Chevron grease noise test, the highest recorded vibrational spike recorded in the medium band during a one-minute run was averaged for five bearings and the average reported as the grease anderon value.
Chevron later refined its test instrument, adding noise pulse counting capability. The pulse counter allows the detection of transients, which are too fast to be recorded on the strip chart recorder. During a test the signal level in each band is displayed on a corresponding meter and is recorded on a strip chart recorder, while the pulse counter detects and displays a figure proportional to the number of vibrational transients that occur above a preset threshold amplitude level. At the end of each test run, the medium band pulse counter reading is noted and the strip chart record of the medium band signal is examined. The first five seconds on the chart are disregarded as start-up noise and the highest amplitude peak (spike) anderon value recorded during the remaining 55 seconds is noted. The noted results for five bearings are averaged and reported as anderon spike value/pulse count.
Chevron further improved its noise testing capability by acquiring the BeQuiet grease noise tester manufactured by SKF Bearing Company. This tester provides additional ability to distinguish subtle differences in noise quality among grease batches. Results are reported in terms of vibrational amplitude in microns/second (similar to anderon value) and in terms of the percentage of measured noise peaks, which fall in to defined noise categories. The noise to categories are designated BQ1, BQ2, BQ3, BQ4, etc. Quieter greases will have a greater percentage of peaks in the lower numbered categories and a lower peak average value.
Different grease compositions have an impact on the amount of bearing vibration and audible noise. Grease noise is attributed to the presence of particles in grease. There are process techniques to help control the particle size during grease manufacture, but these techniques do not improve the low shear stability or heat resistance. In addition to low bearing noise, it is desirable that greases have other properties, including mechanical stability at high and low shear and good heat resistance.
Grease compositions containing a variety of gellant thickeners with urea functional groups have been developed. The polyurea reaction is preferably carried out in situ in the grease carrier, and the reaction product may be utilized directly as a grease.
U.S. Pat. No. 3,243,372 discloses greases thickened with polyureas. In particular, the polyureas have at least four urea groups and hydrocarbon terminal end members.
U.S. Pat. No. 4,436,649 discloses a polyurea-thickened grease containing a polyhydroxylated compound that improves the low shear stability of the grease. The grease composition comprises a major amount of a lubricating oil base vehicle, a polyurea gellant in an amount sufficient to thicken the base vehicle to a grease consistency, and a minor amount of a polyhydroxylated compound.
U.S. Pat. No. 4,661,276 discloses a polyurea-thickened grease containing a polymeric material that improves the low shear stability of the grease. The grease composition comprises a major amount of a lubricating oil base vehicle, a polyurea gellant in an amount sufficient to thicken the base vehicle to a grease consistency, and a minor amount of a polymer having a pKa value greater than 5.0.
U.S. Pat. No. 4,668,411 relates to a diurea type grease composition. The disclosed grease composition comprises a lubricating oil and a thickener, the thickener being a diurea compound prepared by reacting a diisocyanate compound with cyclohexylamine and monoalkylphenylamine wherein the alkyl portion has 8 to 16 carbon atoms.
U.S. Pat. No. 4,780,231 relates to a diurea grease composition containing a lubricant base oil and a thickener. The thickener essentially consists of a mixture of at least two diurea compounds.
U.S. Pat. No. 5,554,586 relates to a grease composition comprising a lubricating oil and a polyurea thickener and a process for its preparation. More specifically, the polyurea thickener is the reaction product of a diisocyanate, a monoamine and a low molecular weight polyoxyalkylene diamine.
U.S. Pat. No. 6,063,743 relates to a lubricating grease composition formed of a basic oil and a lower proportion of a thickening agent which is a polyurea (polycarbamide) compound and the usual additives. These greases were tested and a significant reduction in noise levels was found in comparison with commercially available lubricating grease.
WO 02/04579 relates to a lubricating grease composition having low noise characteristics prepared by shearing a mixture of a base oil and thickener for a time sufficient to reduce substantially all of the thickener to particles below 500 microns in size, and then processing the sheared mixture to a grease. The low noise characteristics are provided by shearing the mixture using a device, such as a static mixer, a mechanical system having counter rotating paddles, cone and stator mills, and roll mills, such that the thickener particles are below 500 microns in size. Therefore, the low noise characteristics are related to the particle size of the thickener.
A common feature of polyurea greases is the way in which they react to shearing (the movement of one lubricant layer with respect to another). At low shear rates, as with simply stirring the grease or working it in a grease worker, the greases tend to soften. In contrast, at high shear rates, as in a rolling element bearing or a grease homogenizer, the greases take on a harder consistency. Generally, this behavior is advantageous for a rolling element bearing grease. However, greases with the tendency to soften under low shear may purge excessively from bearings and cause equipment problems. In order to reduce the extent of low shear softening, grease manufacturers may formulate polyurea greases with the incorporation of a low shear stabilizer.
Relevant literature on the subject of shearing includes Xie Liangsen and Li Hui, Journal of Synthetic Lubrication, Volume 8, No. 1, pages 39-50, which describes the effect of cyclohexyl group on low shear stability. Additional literature references on this subject include C. E. Ward and C. E. Littlefield, NLGI Spokesman, Volume 58, page 178-182 (“Practical Aspects of Grease Noise Testing”); C. E. Ward, “Chevron SRI Grease NLGI 2 and CHEVRON SRI GREASE OEM NLGI 2—A USER'S GUIDE”, 1998; “Lubricating Grease Guide”, Fourth Edition, National Lubricating Grease Institute, ISBN 0-9613935-1-3, Chapters 2-4; and Lube Tips, Noria newsletter, Mar. 15, 2002.
Heat affects grease in several different ways. When exposed to heat, the grease will soften and it may deteriorate due to oxidation. Also, when exposed to heat, components within the grease may evaporate. When heated, grease generally softens and starts to flow readily. Grease usually will not have a sharp melting point. Grease gradually softens at increasing temperatures until it becomes a flowing liquid. A standard test to measure the heat resistance of a grease is the Dropping Point test. The Dropping Point test approximates the end point of the grease softening process. ASTM (American Society for Testing and Materials) D2265 describes this method. Generally, the higher the grease dropping point, the more heat resistant the grease. In this test grease is packed into a standardized thimble or cup, which has a standard hole at the bottom. Most of the grease is removed with a straight rod. A thermometer is then inserted into the cup, and the cup is placed in a standard assembly in a test tube. In carrying out the test, an aluminum block is preheated to a temperature that depends upon the expected dropping point of the grease sample. The test tube-cup assembly is dropped into a hole in the heated aluminum block, and the cup is watched. At some temperature, a drop comes out of the cup from the hole in the bottom and falls to the bottom of the tube. The sample temperature and the block temperature are read immediately. The dropping point is the sum of the sample temperature and one-third the difference between that temperature and the block temperature. Regardless of the nature of the liquid comprising the drop, its presence defines the dropping point.
There remains a need for greases that consistently have low noise characteristics as well as good low shear stability and good heat resistance.